Locking mechanism for bulkheads

ABSTRACT

A freight bracing bulkhead assembly embodying an improved locking mechanism for facilitating release under load. Two embodiments of locking mechanisms are disclosed each of which incorporates a locking member that is slidably supported in a socket opening between a locked position and a released position. A wedging member is also received in the socket opening and precludes transverse movement of the locking member when the wedging member is in a locking position. An operating mechanism is incorporated for first moving the wedging member between its locking position and a released position and subsequently moving the locking member from its locking position to its released position. When the wedging member is in its released position and the locking member is still in its locking position, the locking member and socket member may move in the direction of the load so as to facilitate release of the locking member.

United States Patent [191 [4 1March 13, 1973 Heard LOCKING MECHANISM FOR BULKHEADS [75] Inventor: Robert E. Heard, Plymouth, Mich.

[73] Assignee: Evans Products Company 22 Filed: Jan. 7,1970

[21] Appl. No.: 1,095

[52] U.S. Cl. ..105/376 [51] Int. Cl. ..B60p 7/14 [58] Field of Search ..105/369 A, 369 B, 376; 280/179 A; 248/361 A, 119 R [56] References Cited UNITED STATES PATENTS 3,464,369 9/1969 Erickson et al ..105/376 3,593,674 7/1971 Winterfeldt ....l05/376 3,063,388 ll/l962 Magarian et al.... ....105/376 3,018,741 l/1962 Loomis et a1 ..105/376 3,114,335 12/1963 Schroeder et al.. ....l05/369 B 3,402,680 9/1968 Hyatt ....l05/369 B 3,011,749 12/1961 Kozak ....l05/369 A 891,905 6/1908 Calvert ..280/179 A 3,168,055 2/1965 Vander Hyde et al. ..105/376 3,176,629 4/1965 Shaver ..105/376 Aquino et al ..105/376 Getter ..105/369 A [57] ABSTRACT A freight bracing bulkhead assembly embodying an improved locking mechanism for facilitating release under load. Two embodiments of locking mechanisms are disclosed each of which incorporates a locking member that is slidably supported in a socket opening between a locked position and a released position. A wedging member is also received in the socket opening and precludes transverse movement of the locking member when the wedging member is in a locking position. An operating mechanism is incorporated for first moving the wedging member between its locking position and a released position and subsequently moving the locking member from its locking position to its released position. When the wedging member is in its released position and the locking member is still in its locking position, the locking member and socket member may move in the direction of the load so as to facilitate release of the locking member.

11 Claims, 4 Drawing Figures LOCKING MECHANISM FOR BULKHEADS CROSS-REFERENCE TO RELATED APPLICATION This application is related to the copending patent application of Raymond M. Krokos and John J. Kostrewa entitled Bulkhead Locking Arrangement, Ser. No. 865,277, filed Oct. 19, 1969 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to an improved locking mechanism and more particularly to an improved bulkhead locking mechanism.

In the aforenoted copending patent application of Raymond M. Krokos et al., the difficulties in releasing a conventional locking mechanism of a freight bracing bulkhead assembly is described in detail. Briefly, under extreme loading as occurs when the cargo shifts slightly during transit, the bulkhead frame deflects and considerable load is placed upon the slidably supported locking pins rendering movement of these locking pins to their released position difficult. That application discloses an arrangement for facilitating release of the locking pins under these conditions. The arrangement shown in that application is highly desirable in that it is low in cost. In some instances, however, it may be desirable to provide a more positive means of assuring release of the bulkhead lock pins.

It is, therefore, a principal object of this invention to provide an improved locking mechanism for a freight bracing bulkhead assembly or the like.

It is another object of the invention to provide a bulkhead locking mechanism that facilitates release under high loadings.

It is a further object of the invention to provide a bulkhead locking mechanism embodying a split locking pin arrangement that is sequentially released.

It is still another object of the invention to provide a bulkhead locking mechanism in which certain of the locking pins are released prior to release of other of the locking pins.

SUMMARY OF THE INVENTION A locking mechanism embodying this invention is particularly adapted for use in a freight bracing bulkhead or the like. The locking mechanism includes a socket member, a locking member and a wedging member. The socket member defines an opening in which the locking member is supported for movement between a locked position and a released position. The locking member and socket member have substantial clearance therebetween in a direction normal to the direction of movement of the locking member between its locking and released positions. The wedging member is supported for movement within the socket opening between a locking position and a released position. The wedging member is effective to preclude any substantial movement of the locking member in the normal direction when both the locking and wedging members are in their locking positions. The locking member is movable in the normal direction when the locking member is in its locking position and the wedging member is released. Actuating means are provided for moving the locking member and the wedging member between their locking positions and their released positions.

A further feature of this invention is adapted to be embodied in a freight bracing bulkhead or the like having upper and lower locking pin assemblies. In this connection, an operating mechanism is provided for moving both the upper and lower locking pin assemblies between their locking and released positions. This operating mechanism moves the upper locking pin assemblies to their released positions prior to movement of the lower locking pin assemblies to their released positions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a portion of a railway car or other freight transporting vehicle embodying this invention.

FIG. 2 isan enlarged side elevational view, with a portion broken away, taken generally in the direction of the arrow 2 in FIG. 1.

FIG. 3 is a cross-sectional view taken along the line 33 of FIG. 2.

FIG. 4 is a side elevational view, with portions broken away, in part similar to FIG. 2 and shows another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, a railway freight car or other freight transporting vehicle embodying this invention is identified generally by the reference numeral 11. The car 11 includes a floor 12, roof l3 and opposing side walls 14, only one of which appears in the drawings. A freight bracing bulkhead assembly embodying the invention is identified generally by the reference numeral 15 and is positioned within the car 11 between the side walls 14.

The bulkhead assembly 15 is supported for movement along the length of the car 11 by-means of a trolley arrangement, indicated generally by the reference numeral 16. The trolley arrangement 16 may be of any known type and coacts with a pair of spaced upper supporting rails 17 and 18 that are affixed to the car 11 at the junction of the side walls 14 with the roof 13.

The tracks 17 and 18 are formed with horizontally disposed legs in which locking pin receiving apertures 19 are formed. Lower tracks 21 and 22 are affixed to the floor 12 adjacent its juncture with the side walls 14. The tracks 21 and 22 are also formed with longitudinally spaced pin receiving apertures 23. A locking mechanism, to be described, is carried by the bulkhead assembly 15 for locking the bulkhead assembly in preselected freight bracing positions.

The bulkhead assembly 15 is comprised of a frame structure, indicated generally by the reference numeral 24 and which may be of any known type. A pair of upper locking pins 25 are supported for reciprocation in sockets 26 carried at the upper corners of the frame structure 24. The upper locking pins 25 have a pair of spaced pin projections (not shown) that are adapted to enter adjacent of the pin receiving apertures 19 for locking the bulkhead assembly 15 to the upper tracks 17 and 18.

Lower locking pin assemblies, indicated generally by the reference numeral 27, are carried at the opposite corners of the frame structure 25 for coaction with the tracks 21 and 22. The lower locking pin assemblies 27 will be described in detail by reference to FIGS. 2 and 3. The locking pin assemblies 27 are comprised of a locking pin 28 having a pair of depending pin projections 29 and 31 that are adapted to be received in adjacent of the pin receiving apertures 23.

The locking pin 28 has a pair of oppositely facing surfaces 32 and 33 that extend generally parallel to the load engaging faces of the bulkhead assembly 15. The surface 32 is disposed adjacent a surface 34 of a supporting socket 35 that is affixed to the frame structure 24 in any known manner. The socket surface 34 faces a second socket surface 36 and the surfaces 34 and 36 converge toward the bottom of the frame structure 24. The surfaces 34 and 36 are spaced from each other a greater distance than the spacing between the locking pin surfaces 32 and 33. Thus, except as will be noted, the locking pin 28 is free to move in a direction perpendicular or transverse to the load engaging faces of the bulkhead assembly within the socket 35.

A wedging member, indicated generally by the reference numeral 37, is juxtaposed to the locking pin 28 in the socket 35. The wedging member 37 has a first surface 38 that is juxtaposed to the locking pin surface 33 and which is disposed at substantially the same angle as the surface 33. Opposing the surface 38, the wedging member 37 has a second surface 39 that is disposed at the same angle as the socket surface 36. The wedging member surfaces 38 and 39 converge in a downward direction.

An actuating link 41 is adjustably connected to the wedging member 37 by means of a threaded connection 42 and a pivot pin 43. The actuating link 41 at each side of the bulkhead assembly 15 is connected to a bellcrank 44 by means of a pivot pin 45 (FIG. 1). Also pivotally connected to the bellcrank 44 is an upper actuating link 47 by means of a pivot pin 48. The upper end of the link 47 is connected to the upper locking pin 25 by means of a pivot pin 49. The bellcranks 44 at each side of the bulkhead assembly 15 are affixed to a torsion shaft 51 that is pivotally supported in any known manner on the frame structure 24. A pair of operating handle assemblies 52 and 53 are disposed at opposite sides on the bulkhead assembly 15 for rotating the torsion shaft 51. The handle assemblies 52 and 53 are not shown in any detail and may be of any known type, for example that as shown in the copending patent application of Jackson A. Shook, entitled Freight Bracing Apparatus, Ser. No. 685,980, filed Nov. 27, 1967 issued on Mar. 16,1971 as US. Pat. No. 3,570,416.

Referring again to FIGS. 2 and 3, a pair of short interconnecting members 54 are pivotally connected to the actuating link 41 by means of a pivot pin 55. The opposite ends of the members 54 are pivotally connected to a locking member pin actuating link 56 by means of a pivot pin 57. The actuating link 56 is adjustably connected to the locking pin 28 by means of a screw threaded connection 59 and pivot pin 61. An angularly shaped piece having legs 62 and 63 spans the members 54 and is interposed between the actuating links 41 and 56.

FIGS. 1 through 3 illustrate the bulkhead assembly 15 as being locked in a freight bracing position. Assuming that cargo has been transported within the car 11 to its destination, the cargo will naturally have shifted a large portion of its weight against one of the load engaging faces of the bulkhead frame structure 24. Under this condition, the locking structure of the bulkhead will be exposed to a considerable force due to the deflection of the frame structure 24 and the shear weight of the load. Conventional bulkhead locking structures are extremely difficult to release under this condition. The disclosed arrangement, however, may be conveniently released in the manner now to be described. I

When it is desired to move the bulkhead assembly 15 so that the load may be removed or to reposition the bulkhead assembly 15 in a new freight bracing position, one or the other of the handle assemblies 52 or 53 is manipulated so as to rotate the torsion shaft 51. The bellcranks 44 are thus rotated causing the actuating links 41 and 47 to be drawn upwardly and downwardly, respectively. Downward movement of the upper actuating link 41 causes the locking pins 25 to be withdrawn from the apertures 19 in a known manner.

Referring to FIGS. 2 and 3, when the lower locking assembly 27 is in its locked position, the surface 39 of the wedging member 37 engages the socket surface 36. The wedging member surface 38 snugly engages the locking pin surface 33 and urges the locking pin surface 32 into snug engagement with the socket surface 34. Hence, the wedging member 37 and locking pin 28 are snugly received in the opening of the socket member 35 and the locking pin 28 is held against any transverse movement.

When the actuating link 41 is drawn upwardly, the wedging member 37 is withdrawn to a released position so that its surfaces 39 and 38 are spaced from the socket surface 36 and the locking pin surface 33. During the initial upward movement of the actuating link 41, the members 54 pivot. Initially the end of the leg 63 of the angular shaped member interposed between the members 54 had been in engagement with the actuating link 56 and the full face of the leg 62 was in engagement with the actuating link 41. During the initial movement of the actuating member 41 upwardly, the actuating link 56 does not move and the members 54' merely pivot. This pivotal movement continues until the leg 63 engages the actuating link 41 and the end of the leg 62 engages the actuating link 56. The movement of the actuating link 41 upwardly to move the wedging member 37 toward its released position occurs during the movement of the upper locking pin assemblies 25 from their locking positions to their released positions. Thus, the upper locking pins 25 will have been released prior to any release of the lower locking pins 28.

Once the upper locking pins 25 are released and the members 54 have pivoted, the wedging member 37 will have moved upwardly a sufficient distance to permit relative movement between the locking pin 28 and the socket 35. The wedge shape of the wedging member 37 permits relatively free withdrawal of the wedging member 37 and the relative movement between the socket 35 and locking pin 28 permits the frame structure 24 to shift slightly and relieve the load that has been acting upon the locking structure 27. Continued upward movement of the actuating link 41 is then transmitted through the members 54 into upward movement of the actuating link 56. The locking pin 28 is then drawn upwardly sufficiently so that its pin por- I tions 29 and 31 are free of the apertures 23. At this time, the bulkhead assembly may be moved to a new position.

When the bulkhead assembly 15 is in its new position, the locking mechanism is engaged by rotating the torsion shaft 51 in a direction opposite to that through which it was previously rotated through manipulation of either of the operating handle assemblies 52 and 53. The upper locking pins 25 are engaged in a known manner. When the actuating links 41 move downwardly, the actuating links 56 also move downwardly under their own weight. This downward movement is transmitted to the locking pin 28 so that its pin portions 29 and 31 engage the apertures 23. At this time, a horizontally disposed surface 65 of the pins 28 will engage the upper side of the tracks 21 and 22 halting further downward movement of the locking pins 28. At this time, the wedging member 37 will still be in a released position and its surfaces 38 and 39 will be spaced from the surfaces 33 and 36 of the locking pin 28 and socket 35. During continued downward movement of the actuating link 41, the members 54 will pivot about the pivot pins 57 and the wedging member 37 will continue to move downwardly. This downward movement continues until the surfaces 38 and 39 are in snug engagement with the surfaces 33 and 36 and the locking structures 27 are then in their locked position as previously described.

Referring now to the embodiment of FIG. 4, the overall bulkhead assembly of this embodiment is the same as that previously described. Only the lower locking structure is different and, for this reason, only this structure has been illustrated in FIG. 4 and is identified generally by the reference numeral 71. In the previously described embodiment, the wedging members 37 were disposed on one side of the locking pins 28. The side by side disposition would permit the bulkhead assembly to shift and relieve loads that were exerted upon the bulkhead assemblies on the side adjacent the wedging members 37. No significant shifting of the bulkhead assembly would be permitted, however, if load were exerted on the side adjacent to locking pins 38. In most instances, only one side of the bulkhead assembly engages the freight and this type of construction is satisfactory. In some cases, however, the bulkhead assembly may engage the freight along either of its faces. The embodiment shown in FIG. 4 permits shifting of the bulkhead assembly in either direction relative to. its lower locking pins in a manner which will become apparent as the description proceeds. In this embodiment, a socket opening is defined by a pair of downwardly converging surfaces 72 and 73. A locking pin 74 has converging surfaces 75 and 76 that are disposed at the same angle as the angle of the surfaces 72 and 73. Adjacent the lower termination of the surfaces 75 and 76 a pair of pin projections 77 and 78 are formed, which projections are adapted to be received in adjacent pairs of the track apertures 23.

The locking pin 74 has a narrow upwardly extending projection 79 that is received in the locking position in a cavity 81 formed centrally of a wedging member 82. The wedging member 82 has downwardly converging surfaces 83 and 84 that are disposed at the same angle as the socket surfaces 72 and 73. The surfaces 83 and 84 extend substantially the full length of the socket surfaces 72 and 73 and the surfaces and 76 of the locking pin 74 engage only a small portion of the socket surfaces 72 and 73 when the locking pin 74 and wedging member 82 are both in their locking positions.

A pair of spaced links are connected by means of a pivot pin 86 at their upper end to the wedging member 82. The lower ends of the links 85 are connected to a pivot pin 87 that is received in a slot 88 in the locking pin projection 79 so as to form a lost motion connection between the wedging member 82 and locking pin 74.

FIG. 4 illustrates the locking structure 74 in its locking position. In this position, the surfaces 75 and 76 of the locking pin 74 are snugly engaged with the socket surfaces 72 and 73. It will be noted that the surfaces 74 and 75 are considerably shorter in length than the surfaces 72 and 73. The wedging member surfaces 83 and 84 are also in engagement with the socket surfaces 72 and 73 and the wedging member cavity 81 is snugly engaged with the locking pin projection 79. This engagement precludes any pivotal or rocking move ment of the locking pin 74 in the socket opening and thus holds the locking pin 74 against any significant movement. At this time, the pin 87 is at the lower end of the slot 88 thus further serving to hold the locking pin 74 in position.

In this embodiment, the actuating link 41 is directly connected to the wedging member 82 and when it is desired to release the locking structure 71, the actuating links 41 and wedging member 82 are drawn upwardly. The taper of the surfaces 83 and 84 will facilitate release of the wedging member 82 from its locking to its released position. Initial upward movement of the wedging member 82 will cause the pin 87 to traverse the slot 88 and there will be no significant movement of the locking pin 74. As in the previously described embodiment, this movement occurs during the time that the upper locking pins are moved to their released position.

As the wedging member 82 moves upwardly, its surfaces 83 and 84 will move away from the socket surfaces 72 and 73 and hence release will be facilitated. After the wedging member 82 has moved free of the socket surfaces 72 and 73 any load acting on the bulkhead frame structure 24 can cause the frame structure 24 to shift slightly relative to the locking pin 74 and relieve the loading upon the locking pin 74. Continued upward movement of the actuating link 41 and wedging member 82 will then be transmitted through the pin 87 to the locking pin 74. The pin portions 77 and 78 will then be withdrawn from the apertures 23 and the bulkhead assembly may be moved to a new position.

When in the new position, the locking structure 71 is engaged by moving the actuating link 41 downwardly. At this time, the lock pin 74 will tend to follow the wedging member 82 with the pin 87 remaining at the upper end of the slot 88. As the pin portions 77 and 78 enter the apertures 23 the pin surfaces 75 and 76 will engage the socket surfaces 72 and 73 halting any further downward movement of the locking pin 74. The wedging member 82 will, at this time, continue its downward movement until its surfaces 83 and 84 snugly engage the socket surfaces 72 and 73. During this movement, the pin 87 will traverse the slot 88 in the opposite direction until the projection 79 is engaged by the wedging member 82 and until it reaches the end of this slot. The locking structure 71 will then be in its locked position as previously described.

It should be readily apparent that the lower locking structures of each embodiment embody a locking pin and a wedging member that are sequentially moved between their locking and released positions. In each embodiment, the locking pin moves to its locking position first and is released last. In the locking position, the locking pins are held against any transverse or pivotal movement by the wedging members. During the releasing cycle, the wedging members are released so that the locking pins may undergo relative movement with respect to the bulkhead frame structure so that the frame structure may shift and relieve the load. The locking pins are then freely released. in each embodiment, the tapered surfaces of the wedging members facilitates their release under load.

In each embodiment the movement of the wedging members to their released position is accomplished simultaneously with movement of the upper locking pins to their released positions. This reduces the amount of force necessary to turn the torsion shaft 51 at any given time. That is, since the locking pins are released sequentially, the peak force necessary to turn the torsion shaft is reduced.

WHAT IS CLAIMED IS:

1. A locking mechanism for a freight bracing bulkhead or the like comprising a socket member defining an opening, a locking member supported within said opening for movement between a locked position and a released position, said locking member and said socket member having substantial clearances therebetween in a direction normal to the direction of movement of said locking member between its locking and released positions, a wedging member supported for movement within said socket opening between a locking position and a released position, said wedging member being effective to preclude any substantial movement of said locking member in said normal direction when said locking member and said wedging member are in their locking positions, said locking member being movable in said normal direction relative to said socket member when said locking member is in its locking position and said wedging member is in its released position, and actuating means for moving said locking member and said wedging member between their locking positions and their released positions, said actuating means being effective to sequentially actuate said locking member into its locking position before the movement of said wedging member into its locking position and for moving said wedging member from its locking position to its released position before movement of said locking member to its released position from its locking position.

2. A locking mechanism as set forth in claim 1 wherein the sequential operation of the wedging member and locking member is achieved by means ofa lost motion connection between the wedging member and the locking member.

3. A locking mechanism as set forth in claim 2 wherein the lost motion connection comprises a pin fixed for movement with the wedging member and received in a slot in the locking member.

4. A locking mechanism as set forth in claim 2 wherein the lost motion connection comprises a member pivotally connected to the wedging member and to the locking member and stop means for limiting the degree of pivotal movement of said member in each direction.

5. A locking mechanism as set forth in claim 1 wherein the locking member comprises a locking pin having a pin portion adapted to be received in preselected apertures in a cooperating track.

6. A locking mechanism as set forth in claim 5 wherein the wedging member is disposed in side-byside relationship to the locking pin when the locking pin and wedging member are in their locking positions.

7. A locking mechanism as set forth in claim 1 wherein the wedging member has inclined surfaces for facilitating its release under load.

8. A locking mechanism as set forth in claim 7 wherein one of the inclined surfaces of the wedging member engages a cooperating surface of the locking member, the other inclined surface of the wedging member being adapted to engage an inclined surface of the socket member.

9. A locking mechanism as set forth in claim 7 wherein the inclined surfaces of the wedging member are adapted to engage cooperating inclined surfaces of the socket member.

10. A locking mechanism as set forth in claim 1 in combination with a freight bracing bulkhead assembly, said freight bracing bulkhead assembly further including a second locking member supported for movement between a locking position and a released position, and operating means for moving said second locking member from its locking position to its released position prior to movement of the first mentioned locking member from its locking position to its released position.

11. A bulkhead assembly as set forth in claim 10 wherein the operating means moves the second locking member between its locking position and its released position simultaneously with movement of the wedging member between its locking position and its released position. 

1. A locking mechanism for a freight bracing bulkhead or the like comprising a socket member defining an opening, a locking member supported within said opening for movement between a locked position and a released position, said locking member and said socket member having substantial clearances therebetween in a direction normal to the direction of movement of said locking member between its locking and released positions, a wedging member supported for movement within said socket opening between a locking position and a released position, said wedging member being effective to preclude any substantial movement of said locking member in said normal direction when said locking member and said wedging member are in their locking positions, said locking member being movable in said normal direction relative to said socket member when said locking member is in its locking position and said wedging member is in its released position, and actuating means for moving said locking member and said wedging member between their locking positions and their released positions, said actuating means being effective to sequentially actuate said locking member into its locking position before the movement of said wedging member into its locking position and for moving said wedgIng member from its locking position to its released position before movement of said locking member to its released position from its locking position.
 1. A locking mechanism for a freight bracing bulkhead or the like comprising a socket member defining an opening, a locking member supported within said opening for movement between a locked position and a released position, said locking member and said socket member having substantial clearances therebetween in a direction normal to the direction of movement of said locking member between its locking and released positions, a wedging member supported for movement within said socket opening between a locking position and a released position, said wedging member being effective to preclude any substantial movement of said locking member in said normal direction when said locking member and said wedging member are in their locking positions, said locking member being movable in said normal direction relative to said socket member when said locking member is in its locking position and said wedging member is in its released position, and actuating means for moving said locking member and said wedging member between their locking positions and their released positions, said actuating means being effective to sequentially actuate said locking member into its locking position before the movement of said wedging member into its locking position and for moving said wedgIng member from its locking position to its released position before movement of said locking member to its released position from its locking position.
 2. A locking mechanism as set forth in claim 1 wherein the sequential operation of the wedging member and locking member is achieved by means of a lost motion connection between the wedging member and the locking member.
 3. A locking mechanism as set forth in claim 2 wherein the lost motion connection comprises a pin fixed for movement with the wedging member and received in a slot in the locking member.
 4. A locking mechanism as set forth in claim 2 wherein the lost motion connection comprises a member pivotally connected to the wedging member and to the locking member and stop means for limiting the degree of pivotal movement of said member in each direction.
 5. A locking mechanism as set forth in claim 1 wherein the locking member comprises a locking pin having a pin portion adapted to be received in preselected apertures in a cooperating track.
 6. A locking mechanism as set forth in claim 5 wherein the wedging member is disposed in side-by-side relationship to the locking pin when the locking pin and wedging member are in their locking positions.
 7. A locking mechanism as set forth in claim 1 wherein the wedging member has inclined surfaces for facilitating its release under load.
 8. A locking mechanism as set forth in claim 7 wherein one of the inclined surfaces of the wedging member engages a cooperating surface of the locking member, the other inclined surface of the wedging member being adapted to engage an inclined surface of the socket member.
 9. A locking mechanism as set forth in claim 7 wherein the inclined surfaces of the wedging member are adapted to engage cooperating inclined surfaces of the socket member.
 10. A locking mechanism as set forth in claim 1 in combination with a freight bracing bulkhead assembly, said freight bracing bulkhead assembly further including a second locking member supported for movement between a locking position and a released position, and operating means for moving said second locking member from its locking position to its released position prior to movement of the first mentioned locking member from its locking position to its released position. 